Light-weight magnesium alloys having excellent specific strength and specific rigidity are being studied as constituent materials of parts, such as housings for mobile electronic devices, including cellular phones and laptop computers. Magnesium alloy parts are mainly made of cast materials manufactured by a die-casting process or a thixomold process (AZ91 alloy as defined in the American Society for Testing and Materials standards). In recent years, parts manufactured by press forming of a sheet made of a wrought magnesium alloy exemplified by AZ31 alloy as defined in the American Society for Testing and Materials standards have been used. Patent Literatures 1 and 2 disclose press forming of a rolled sheet manufactured under particular conditions from AZ91 alloy or an alloy that has substantially the same Al content as AZ91 alloy.
Magnesium alloys generally have low corrosion resistance. Thus, as disclosed in Patent Literature 1, magnesium alloys are subjected to surface treatment, such as chemical conversion treatment or anodizing treatment, to improve corrosion resistance. Furthermore, the compositions of magnesium alloys may be adjusted to improve corrosion resistance. For example, AZ91 alloy having a higher Al content than AZ31 alloy has excellent corrosion resistance.